Currently, the problem of searching for unmapped metalliferous objects, including objects with nonferrous metals, has become most urgent. One approach to solving this problem is the use of compact inductive devices combining the functions of radiating an original electromagnetic field and reception of a secondary electromagnetic field induced by the metalliferous object positioned within the range of the original electromagnetic field.
Known in the art are detectors of buried metalliferous objects (high-frequency FM 480 finder from Seba Dynatronic, FRG, and pipeline and wire finder TW=5 from Grigsby Co., USA), of similar design and each comprising: a generator loaded onto a driving loop, a receiving loop orthogonal to the driving loop and loaded by connected in series selective amplifier, data processor and acoustic signalling device. The detector is also equipped with a geometric means to cancel the signal induced by the original electromagnetic field of the driving loop in the receiving loop.
These detectors function by radiating an original electromagnetic field from the driving loop into the area under study and picking up a secondary electromagnetic field induced by the metalliferous object.
These detectors are highly sensitive to external electromagnetic fields and to interference from utility systems located in the vicinity of the search site, because they also radiate a secondary electromagnetic field under the effect of the original electromagnetic field of the driving loop.
Furthermore, the reliability of detecting utility systems positioned at angles less than 20.degree. relative to the direction of receiving loop movement is rather low, due to the low level of signal induced by the driving loop in the utility system.
Also widely known in the art are metalliferous objects detectors (V. F. Bakhmutsky, G. I. Zuenko "Induktsyonnye kabeleiskateli" (Induction Wire-Finders), 1970, Moscow, Svyaz Publishers, pp. 75-78, FIGS. 31-32. In Russian), each comprising a driving loop and two receiving loops rigidly tied to it, orthogonally positioned and differentially connected, located symmetrically to both sides of the driving loop. In one case the driving loop is positioned vertically and the receiving loops are horizontal, in another case loop orientations are reversed. The electric signal recording circuitry in bothcases are identical and similar to that described herein above.
It should be noted, that the direction of detector movement coincides with that of the rod carrying the receiving loops.
Such detectors are prone to interference from external electromagnetic fields and metalliferous objects in the vicinity of the search site. This is due to the considerable spacing between the receiving loops, necessary to ensure an adequate sensitivity and adequate degree of geometric cancelling. However, at a large spacing between receiving loops and at utility systems positioned at an angle less than 20.degree. relative to the direction of detector movement, the detection reliability is poor because of the low intensity of the secondary electromagnetic field from the utility system at vertical receiving loops or low signal induced in the utility system at horizontal receiving loops.
It should be noted, that due to the poor interference immunity of the herein above cited detectors, they cannot be used when mounted on transportation means, which would itself be a source of interference, so that searches have to be performed manually, this impairing productivity.
The metalliferous objects detector (SU, A, 1190330) features a higher productivity and higher immunity to external electromagnetic fields and comprises a driving loop rigidly tied to two receiving loops positioned coaxially and in parallel to one another and symmetrically and orthogonally relative to the driving loop positioned in the plane of the common axis of the receiving loops, an AC voltage generator electrically connected to the driving loop, and connected in series balancer to provide balancing of the signals from the receiving loops and with inputs connected to the receiving loops, and differential amplifier with the output thereof connected to the data input of an electric signal compensator intended to cancel the effect of the original electromagnetic field of the driving loop on the receiving loops and having a control input connected to the AC voltage generator and with the output thereof driving the input of a selective amplifier, the output whereof is connected to the input of a data processor with the output thereof connected to a recorder and electrically coupled to a signalling unit.
The mutual arrangement of the driving and receiving loops herein above described provided a substantial improvement in the detector's interference because external electromagnetic fields (from power distribution lines, electrified railways, communications lines, etc.) induce practically identical signal levels in both receiving loops, and these signals are mutually cancelled out due to their opposite connection.
However, a metalliferous object (utility system) positioned at an angle less than 20.degree. relative to the search direction, which is normal to the rod carrying the receiving loops, the detection reliability is poor due to the low intensity of the electromagnetic field induced by the driving loop in the metalliferous object.
To improve the detection probability when searching for utility systems one and the same ground has to be covered twice, at different orientation of the induction system relative to the direction of movement. However, even this does not ensure adequate detection probability because it is difficult to maintain the same search direction when moving in the opposite direction and this is true of both manual and mechanized searching.